My comment is actually very simple.
Much of Go is about connection and cutting. A connected group is often a strong group, while disconnected groups can be weak and vulnerable to attacks.
As a rule of thumb, it is recommended to keep your stones connected, and cut your opponent's stones.
The black move I singled out simply violates this rule of thumb to the extreme, by inviting white to cut black into two weak groups while maintaining a single connected white chain.
As it happens, I wrote about this topic slightly more than a decade back. I believe what I wrote still stands today.
http://yeefan.sg/weiqi/howtoplaygo/?lesson=04
Also, Charles Matthews also wrote about this on Sensei's Library:
http://senseis.xmp.net/?SplitShape